In a production well in a well bore, the amount of production from the well can be optimized if the downhole pressure at or near the producing formations is known. To obtain downhole pressure measurements in a production well, it has been heretofore a practice to dispose a production string of tubing in a well bore with an attached pressure gauge at the lower end of the production string of tubing. The pressure gauge has access to the bore of the production tubing for sensing pressure in the tubing and an exterior conductor cable couples the pressure sensing device to the earth's surface for a determination and monitoring of the pressure in the bore of the string of tubing. With the knowledge of pressure in the downhole string of tubing, the operator can optimize the flow of production and adjust the production rate to the downhole pressures for optimum production conditions.
In permanent installations where a pressure gauge is installed, the production of the well may occur over an extended period of time, which may be months into years before the production tubing is removed, if removed at all. Thus, it is necessary that a pressure gauge for use in a permanent installation have a long life and reliability because it is often times impractical and too expensive to remove a production string to replace a failed pressure gauge.
In deep well bores particularly, the downhole temperature increases as a function of depth and the temperatures typically are greater than 100.degree. C. Temperatures above 100.degree. C. introduce special problems for pressure gauges typically employed for measurement of downhole pressure. This is because a high temperature adversely affects the electronics in the components in a pressure gauge and will accelerate failure and wear out of the electrical components. Obviously, the likelihood of a system failure increases as a function of the number of electronic components in the tool as well as the ambient temperature.
In an effort to solve the problem of gauge failure, it has been proposed to use redundant downhole pressure systems. That is, two completely separate pressure gauge systems are used to increase the reliability factor. However, the use of redundant downhole pressure systems creates a degree of difficulty and expense of installing two separate cables to the earth's surface which can be prohibitive and in some cases impossible to obtain. Two pressure gauge systems could be connected to a single cable if the transmission of pressure measurements is alternated. With alternate transmission, only one pressure sensor gauge system would communicate at a time with the cable and there would be no interference from one pressure sensor gauge system to another. This approach, however, would require synchronization of the electronics in the pressure gauge system and require interconnection which limits the pressure gauge system to a common housing.
In the present invention the reliability of a downhole pressure system for high temperature wells is increased by reducing the number of electronic components which are exposed to the environment. Additionally, the present invention contemplates reducing power dissipation in the downhole tool to minimum since power dissipation in downhole components only increases the operating temperature above ambient and aggravates the temperature exposure problem of electronic components. Multiple independent pressure gauge systems can be used with a single wire cable.